Co-production of fuels, chemicals and electric power using turbochargers

ABSTRACT

A method and system for co-production of electric power, fuel, and chemicals in which a synthesis gas at a first pressure is expanded using a turbo-expander, simultaneously producing electric power and an expanded synthesis gas at a second pressure after which the expanded synthesis gas is converted to a fuel and/or a chemical.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method and system for co-producing fuels,chemicals, and electric power. In one aspect, this invention relates tothe use of mechanical expanders and gas turbines for co-producing fuels,chemicals, and electric power. In one aspect, this invention relates tothe use of turbo-compressors and turbo-expanders, collectivelyturbochargers, for co-producing fuels, chemicals, and electric power. Inone aspect, this invention relates to the use of turbochargers for theproduction of synthetic gas, also referred to as syngas, and electricpower, and the subsequent conversion of the syngas to various liquidfuels and/or chemicals.

2. Description of Related Art

There are a variety of known processes for the generation of synthesisgas. U.S. Pat. No. 6,306,917 B1 to Bohn et al. teaches a method andapparatus for producing power, liquid hydrocarbons and carbon dioxidefrom heavy feedstocks using a partial oxidation reactor to producesynthesis gas, a Fischer-Tropsch (FT) reactor to convert the synthesisgas to hydrocarbon products and tail gases containing hydrogen andcarbon dioxide, and a combined cycle plant to produce power from steamgenerated by recovering heat from the reactors and from combustible tailgases. U.S. Pat. No. 6,596,780 B2 to Jahnke et al. teaches a method forgenerating syngas comprising H₂ and CO₂ by gasification ofhydrocarbonaceous fuels, such as coal, oil or gas, scrubbing the syngasfree of particles, and saturating with water. The syngas is then treatedin an acid gas removal unit as desired to remove any impurities in thesyngas after which it is routed to a hydrocarbon synthesis reactor inwhich the H₂ and CO₂ in the syngas are converted to synthetichydrocarbons. The unreacted tailgas exiting the reactor is sent to a gasturbine as fuel.

Gas turbines are one of the major sources for power generation in usetoday. However, the best efficiency achieved to date using simple cyclegas turbines is only about 38%. One significant drawback of gas turbinesis that a significant portion of fuel energy input to the gas turbines,approximately 62-75%, is lost in the turbine exhaust. This exhaustenergy is in the form of thermal energy only, which makes it difficultto use for effective power generation. Staged reheat gas turbines havethe capability to improve both efficiency and NO_(x) emissions. In somegas turbines, fuel staging has been employed. Fuel staging improvessystem efficiency but has limited application due to combustioninstability problems, particularly in the first stage, high NO_(x)emissions, and a large portion of thermal energy, about 55-65%, in theturbine exhaust. U.S. Pat. No. 7,421,835 B2 to Rabovitser et al. teachesa two-stage power generation system having a compressed air source withtwo compressed air outlets, one of which provides compressed air to thefirst stage of power generation and the other of which providescompressed air to the second stage of power generation. All of the fuelfor the two-stage power generation system is introduced into the firststage. Exhaust gases from the first stage are introduced into a fuelinlet of the second stage of power generation. The first stagepreferably includes a gas turbine operated in partial oxidation mode.The exhaust gases from the partial oxidation gas turbine contain thermaland chemical energy, both of which are used in the second stage. Inaccordance with one embodiment, the exhaust gases are split into twostreams, one of which is employed for power generation and the other ofwhich is used for hydrogen production.

A turbocharger, which comprises a turbo-compressor and a turbo-expanderwherein the turbo-expander drives the turbo-compressor, typically isused to improve the volumetric efficiency of an engine by increasing theintake density. The turbo-compressor draws in ambient air and compressesit before it enters into the engine intake manifold at increasedpressure, resulting in a greater mass of air entering the cylinders oneach intake stroke. The power needed to operate the turbo-compressor isderived from energy associated with the high temperature and pressure ofthe engine's exhaust gases. The turbo-expander converts the engineexhaust's thermal and pressure energy, which are transformed intokinetic velocity energy and then into rotational power which, in turn,is used to drive the turbo-compressor.

For conventional syngas generation processes such as steam methanereforming (SMR), shown in FIG. 7, and autothermal reforming (ATR), shownin FIG. 8, involving the reactions of natural gas with oxygen and steamthat uses a catalytic reactor or a non-catalytic partial oxidation(PO_(x)) reactor, the outlet temperature of the syngas is limited,typically to about 1600-1700° F. at pressures of about 20-40 atm, due tolimitations of the metallurgy for the equipment such as a waste heatboiler for production of steam used in the syngas cooling step, and dueto the catalysts used for reforming reactions.

SUMMARY OF THE INVENTION

It is, thus, one object of this invention to provide a method and systemfor co-producing fuels, chemicals, and electric power which overcomesthe temperature limitations dictated by the metallurgy of the equipmentused to make waste heat boilers and similar systems or devices forcooling the syngas.

It is another object of this invention to provide a method and systemfor co-producing fuels, chemicals, including hydrogen, and electricpower which enables the generation of syngas at pressures higher thanonly about one atmosphere.

It is yet another object of this invention to provide a method andsystem for co-producing fuels, chemicals, and electric power in whichthe syngas pressure and temperature produced in the method are such asto reduce the net compression energy requirements for using the exitingsyngas in downstream gas-to-liquid or gas-to-chemicals conversionprocesses.

It is yet another object of this invention to provide a method andsystem for co-producing fuels, chemicals, and electric power in whichthe composition of the syngas produced in the method is controlled so asto provide compositions favorable for liquids or chemicals production.

These and other objects of this invention are addressed by a method andsystem for co-producing fuels, chemicals, and electric power utilizing aturbocharger in which oxidant is compressed by the turbo-compressor andsubsequently provided to a reactor vessel in which a synthesis gas isgenerated, producing pressurized synthesis gas at a first pressure,which pressurized synthesis gas is expanded by the turbo-expander,simultaneously producing electric power and an expanded synthesis gas ata second pressure, all the while driving the turbo-compressor. Theexpanded synthesis gas is then converted to a fuel and/or a chemical. Incontrast to a conventional gas turbine having a closed-coupled aircompressor and expander system, the use of a turbocharger facilitatesthe generation of syngas at pressures substantially higher than aboutone atmosphere by employing significantly high back-pressures at theexit to the turbo-expander without the added complexities associatedwith integrating a fuel/air combustor with a conventional gas turbine.For syngas production using natural gas, the volume (or total number ofmoles) of syngas is higher than that of the natural gas feed plus theoxidants (steam or oxygen), as a result of which the turbo-expander ofthe turbocharger can be operated with significantly high back pressure,making it more cost-effective to use relatively high pressure feednatural gas (with oxidants) in the syngas production and minimal energy(as necessary) for any required downstream syngas compression.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of this invention will be betterunderstood from the following detailed description taken in conjunctionwith the figures, wherein:

FIG. 1 is a diagrammatic representation of a method and system forco-production of fuels and electric power in accordance with oneembodiment of this invention employing a turbo-compressor andturbo-expander with an autothermal or steam methane reformer for syngasgeneration;

FIG. 2 is a diagrammatic representation of a method and system forco-production of fuels and electric power in accordance with anotherembodiment of this invention employing a turbo-compressor andturbo-expander with coal or biomass gasification for syngas generation;

FIG. 3 is a diagrammatic representation of a method and system forco-production of fuels and electric power in accordance with oneembodiment of this invention in which the syngas is produced in apartial oxidation reactor and a turbo-expander is used for syngasexpansion and power generation;

FIG. 4 is a diagrammatic representation of a turbocharger-based processfor the co-production of Fischer-Tropsch (FT)-type liquid fuels andelectric power from natural gas in accordance with one embodiment ofthis invention;

FIG. 5 is a diagrammatic representation of a turbocharger-based processfor the co-production of gasoline-type liquid fuels and electric powerfrom natural gas in accordance with one embodiment of this invention;

FIG. 6 is a diagrammatic representation of a method and system forco-production of ammonia and electric power in accordance with oneembodiment of this invention;

FIG. 7 is a diagrammatic representation of a conventional steam methanereforming (SMR) process for the production of syngas from natural gasand steam; and

FIG. 8 is a diagrammatic representation of a conventional autothermalreforming process for the production of syngas from natural gas andoxygen/steam.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The invention claimed herein is a method and system for co-producingfuels, chemicals, and electric power in which compressed oxidantproduced by a turbo-compressor is provided to a reactor vessel in whichpressurized synthesis gas at a first pressure is produced andsubsequently expanded using a turbo-expander, simultaneously producingelectric power and an expanded synthesis gas at a second pressure whichis converted to a fuel and/or a chemical. The synthesis gas, typicallycomprising substantial amounts of H₂ and CO, may be produced by anymeans known to those skilled in the art including gasification ofcarbonaceous materials such as coal and biomass, steam reforming (FIG.7) or autothermal reforming (ATR) (FIG. 8) of natural gas or otherprocess streams containing significant amounts of methane, ethane,propane, butane and related C₂-C₄ olefinic hydrocarbons, partialoxidation, and catalytic partial oxidation. Various oxidants and oxygencarriers including oxygen-enriched air, oxygen, air, CO₂, steam, andmixtures thereof may be employed. The use of CO₂ is particularlyadvantageous due to its higher molecular weight. In addition, due to thehigher molecular weight, CO₂ has better heat transfer properties.

In accordance with one preferred embodiment of this invention, thesynthesis gas is produced from natural gas. Chemical reactions for theproduction of syngas from natural gas include:

CH₄+0.5O₂→CO+2H₂

CH₄+CO₂→2CO+2H₂

CH₄+H₂O→CO+3H₂

CO+H₂O→CO₂+H₂

As used herein, the term “natural gas” (NG) refers to gases in which theprimary component is methane including conventional natural gas,associated natural gas, stranded natural gas, biogas, landfill gas, andbyproduct gases from various industrial processes. In accordance withone preferred embodiment of this invention, the synthesis gas isproduced by the partial oxidation of natural gas in a partial oxidationreactor (POR) using an oxidant and oxygen carrier mixture undersub-stoichiometric conditions.

FIG. 1 is a schematic diagram showing the primary steps of the method inaccordance with one embodiment of this invention in which natural gasand steam and compressed oxidant produced by a turbo-compressor 13 areprovided to a reforming reactor vessel 10 in which the natural gas isconverted to syngas, either by means of an autothermal reforming process(ATR) or a steam methane reforming process (SMR). FIG. 2 is a schematicdiagram showing the primary steps of the method in accordance with oneembodiment of this invention in which the syngas is generated in aconventional coal or biomass gasification reactor 20 following which thesyngas is provided to a syngas cleaning process reactor 21, such as forsulfur removal. Depending on the specific syngas generation processemployed and the related limitations on the maximum outlet temperatureof the syngas product, and depending on the overall utility requirementsfor the production of liquid fuels and chemicals, a booster combustor11, fueled by a portion of the syngas and additional oxidant, may beused for increasing the syngas temperature prior to expansion of thesyngas. The temperature of the syngas prior to expansion is preferablyin the range of about 2200° F. to about 2500° F. The syngas generated bythe reforming process is output from the process at a first pressure,P₁, in the range of about 60 psig to about 1500 psig, preferably greaterthan about 800 psig.

In accordance with one embodiment of this invention, the syngas isproduced in a coal or biomass gasifier. For example, in biomassgasification, methane-rich syngas from a typical catalytic tar reformermay first be sent to a hot filter unit to remove residual fine solids,and the effluent from the filter may be sent to a non-catalytic PORunit. The effluent from this non-catalytic POR may then be sent to anexpander unit for (i) generating electricity and (ii) cooling thesyngas. In this way, the use of a conventional “waste heat boiler” or aheat exchanger to cool the syngas from the hot filter may be avoided; inaddition, this will allow conversion of a large fraction of the residualmethane (and ethane/propane) to syngas. In some cases, the tar reformermay be eliminated and the syngas from the gasifier may be processed in ahot filter and in a non-catalytic partial oxidation reactor unit.

The hot syngas from the syngas generation step is introduced, inaccordance with one embodiment of this invention, into a turbo-expander12, which is coupled with the turbo-compressor 13 for driving theturbo-compressor, in which the syngas is expanded to co-produce electricpower and a relatively cooled syngas, typically at a temperature in therange of about 900° F. to about 1200° F. and at a second pressure, P₂,which is less than the first pressure, P₁, but which is significantlyhigher than one atmosphere. Following expansion, the cooled syngas maybe subjected to additional processing in a syngas conditioning process14, such as for water and CO₂ removal, and/or for syngas compositionmodification 15, e.g. by way of a shift reaction to increase the H₂/COratio or H₂ removal. Thereafter, the “conditioned” syngas is compressedby a syngas compressor 17 and the compressed syngas may be provided to asyngas-to-liquids processing step 18 in which the syngas is converted toa liquid fuel, such as diesel, gasoline, ethanol or LPG, or theconditioned syngas may be provided to a syngas-to-chemical processingstep in which the syngas is converted to a chemical, such as hydrogen,ammonia, methanol, etc.

FIG. 3 is a schematic diagram of a method for co-producing fuels,chemicals, and electric power in accordance with one embodiment of thisinvention employing a partial oxidation reactor for generation of thehot syngas. In accordance with this embodiment, the H₂/CO molar ratio inthe syngas from the expander is about 0.69, typical of those achieved incoal gasification.

As shown in FIG. 3, a mixture of CO₂ and O₂, the components of which maybe provided by any suitable source, such as (1) CO₂ recovered, forexample, from power plants, and (2) a vacuum pressure swing adsorption(VPSA) unit or a cryogenic air separation unit (ASU) for supplying O₂,is used as the oxidant. The mixture is compressed in turbo-compressor13, producing a compressed mixture which is provided together with afuel to a partial oxidation reactor 32 at sub-stoichiometric conditions,in which the fuel is partially oxidized, producing a hot syngas. The hotsyngas is provided to turbo-expander 12 in which the hot syngas isexpanded, producing a cooler syngas and electric power. Depending on thedownstream requirements, the cooler syngas may be subjected to any of anumber of conditioning steps, which may include the use of conventionalheat exchanger, heat recovery steam generators, and thermochemicalrecuperation, for conditioning the syngas to satisfy the downstreamrequirements. In accordance with one embodiment, the cooler syngas isintroduced into a thermochemical recuperator (TCR) 34 in which thesyngas is further cooled to provide heat for the steam reforming of aportion of the natural gas fuel. The partially reformed natural gasstream (containing some H₂ and CO) from the reforming reaction is thenfed to the partial oxidation reactor 32. The cooled syngas from the TCRis then processed in a shift reactor 36 in which the ratio of H₂/CO inthe syngas is increased and the temperature of the syngas is increased.Thermochemical recuperation increases the overall system efficiency byconverting a part of the heat energy into chemical energy (including viaconversion of CO₂ and CH₄ to syngas). The effluent syngas from the shiftreactor 36 is provided to a heat recovery steam generator (HRSG) 35, thesteam from which is provided to a steam turbine 40, resulting in thegeneration of additional electric power. The steam from the steamturbine is split by a suitable splitting means 41 into four streams, oneof which is mixed with a fuel, producing a fuel/steam mixture which isprovided to thermochemical recuperator 34, another of which is providedto the shift reactor 36, another of which is provided to theturbo-expander 33, and the last of which is provided to the partialoxidation reactor 32.

Alternatively, the cooler syngas may be conditioned by using a water-gasshift reactor 36 to increase the H₂/CO ratio, by removing water vaporand CO₂, and by syngas compression. The conditioned syngas then may beintroduced into a suitable syngas-to-liquids process, e.g. Velocys FTdiesel, or ExxonMobil MTG or Haldor-Topsoe TIGAS gasoline, or a chemicalproduction process (e.g., ammonia, methanol). As shown in FIG. 3, thecooled syngas is conditioned for use in the production of liquid fuelsin a Fischer-Tropsch plant 50. Tail gas from the Fischer-Tropsch plantis then provided to a bottoming cycle 37 for generation of additionalelectric power.

In accordance with one embodiment of this invention, prior to partialoxidation of the natural gas, the natural gas stream may be desulfurizedto meet the requirements of downstream utilization, such as in asyngas-to-liquids process which requires syngas with very low levels ofsulfur compounds.

In a conventional gas turbine operation, a large amount of excess air isrequired to ensure complete combustion of the natural gas fuel and tomeet specific temperature limitations at the inlet of theturbo-expander. In accordance with one embodiment of this invention, theair/fuel ratio is reduced significantly by reducing the amount of O₂/airand by increasing the amount of natural gas to attain specificsub-stoichiometric (partial oxidation) combustion conditions. Thetemperature at the outlet of the partial oxidation reactor is preferablymaintained in the range of about 2,200-2,500° F. However, withimprovements in the metallurgy of the turbo-expander, this temperaturemay be increased accordingly.

In accordance with one embodiment of this invention, the partialoxidation reactor comprises two separate sections, an initial sectionwhich may be filled with suitable catalysts to enhance reaction kineticsfor conversions of the hydrocarbons to CO and H₂ via reactions withoxygen and a final section which may be non-catalytic. Depending on thedesired compositions of product liquid fuels and chemicals, the levelsof various oxidants (O₂, air, steam, CO₂) may be controlled to achieverequired levels of H₂/CO ratios and nitrogen in the effluent syngas.

Following production of the syngas in the partial oxidation reactor inaccordance with one embodiment of this invention, the hot syngas isexpanded in a turbo-expander to co-produce electric power and relativelycooled syngas (typically, at temperatures in the range of about900-1,200° F.). Typically, for a conventional gas turbine, the inletpressure of hot gas at the inlet of the turbo-expander is about 200-250psig with an outlet pressure of about 15 psia. Depending on the streampressure of available natural gas feed, the required pressure for thesyngas feed to a downstream liquid fuels production unit (e.g. aFischer-Tropsch liquids production facility in which the required syngasfeed pressure is about 350-450 psig), and specific mechanical design ofthe turbo-expander, the expander is operated with a similar pressuredrop (e.g. about 200-250 psia) for the syngas stream, thereby enabling areduction in the cost of the syngas compressor unit.

FIG. 4 is a schematic diagram showing the co-production of liquid fuelsand electric power in accordance with one embodiment of this invention.The system and method for generating the syngas is substantially thesame as the system and method shown in FIG. 3 except that a portion ofthe mixture of CO₂ and O₂ used as oxidant and a portion of the steamproduced in a heat recovery steam generator (HSRG) are provided to asecond partial oxidation reactor 60 for conversion of a portion of thehydrocarbons produced in the Fischer-Tropsch plant with enriched O₂ andsteam to produce additional syngas that may be recycled to theFisher-Tropsch reactor. The FT block may include other processing stepssuch as hydrocracking of wax made in the FT reactor.

Similarly, FIG. 5 is a schematic diagram of a methanol-to-gasoline (MTG)type syngas-to-liquid production process in accordance with oneembodiment of this invention for which the H₂/CO ratio should be about2.0. The syngas (with a total H₂+CO mol % level at ˜55.3%) from theturbo-expander would contain a H₂/CO ratio of about 1.66, which may beincreased to about 2.0 by using a shift reactor as shown in FIG. 5. Thesyngas pressure to the MTG section would be about 800-1500 psig; thusadditional syngas compression would be required to operate the MTGplant. There are other technologies, such as the Haldor Topsoe TIGAStechnology, for the production of gasoline from syngas in which theH₂/CO ratio requirements could be considerably less than 2.0.

FIG. 6 is a schematic diagram for a system to co-produce a chemical,e.g. ammonia, and electric power in accordance with one embodiment ofthis invention. The key chemical reaction is: 3H₂+N₂→2NH₃. For thissystem, it would be necessary to increase the syngas pressure to about200 atm before the ammonia reactor.

As previously indicated, the turbo-expander may be used to cool thesyngas through the production of electric power. With continuedimprovements in expander designs, the inlet temperature can besignificantly higher, on the order of 2300-2400° F. at pressures ofabout 200-250 psig, than those which can be achieved using conventionalsteam methane reforming or ATR/PO_(x). For higher pressure operations,e.g. at inlet pressures of 600 psig and effluent pressures of about 350psig for a given gas turbine, experimentation would be required todetermine the upper limit on the inlet gas temperatures.

One key advantage of this invention is that it can be used to produce aconcentrated CO₂ stream for sequestration or recycle to the front end ofthe process. This is similar to the ATR or PO_(x) designs. However, inthe case of a SMR, the endothermic heat of reaction for reformingreactions is supplied to tubes filled with catalysts indirectly in afurnace-type design (FIG. 7). In this case, the CO₂ level in the fluegas is relatively low, making CO₂ recovery rather difficult andexpensive.

The use of a turbocharger for co-producing syngas as well as powersignificantly reduces net foot print need and overall capital/operatingcost requirements and increases net thermal efficiencies for theproduction of liquid fuels and chemicals from natural gas, especiallyfor small-scale plants.

While in the foregoing specification this invention has been describedin relation to certain preferred embodiments thereof, and many detailshave been set forth for the purpose of illustration, it will be apparentto those skilled in the art that the invention is susceptible toadditional embodiments and that certain of the details described hereincan be varied considerably without departing from the basic principlesof this invention.

1. A method for co-production of fuels, chemicals, and electric powercomprising the steps of: compressing an oxidant using aturbo-compressor, producing compressed oxidant, and introducing saidcompressed oxidant into a reactor vessel; generating a synthesis gas insaid reactor vessel, producing pressurized synthesis gas at a firstpressure and a first temperature; expanding said pressurized synthesisgas using a turbo-expander, simultaneously producing electric power andan expanded synthesis gas at a second pressure and a second temperature;and converting said expanded synthesis gas to at least one of a fuel anda chemical.
 2. The method of claim 1, wherein said second pressure isless than said first pressure.
 3. The method of claim 2, wherein saidsecond pressure is greater than about 1 atm.
 4. The method of claim 1,wherein said oxidant is mixed with CO₂.
 5. The method of claim 4,wherein a CO₂/O₂ molar ratio is in the range of about 0.1 to about 2.0.6. The method of claim 5, wherein said CO₂/O₂ molar ratio is in therange of about 0.9 to about 1.1.
 7. The method of claim 1, wherein saidoxidant is selected from the group consisting of air, oxygen,oxygen-enriched air, steam, and mixtures thereof.
 8. The method of claim1, wherein said converting of said expanded synthesis gas produces atail gas.
 9. The method of claim 8, wherein said tail gas is employed ina bottoming cycle.
 10. The method of claim 1, wherein said firstpressure is greater than about 60 psig.
 11. The method of claim 10,wherein said first pressure is greater than about 800 psig.
 12. Themethod of claim 1, wherein said reactor vessel is a partial oxidationreactor vessel.
 13. The method of claim 12, wherein said fuel is aFischer-Tropsch liquid fuel.
 14. The method of claim 13, wherein aportion of said liquid fuel is introduced into a second partialoxidation reactor and converted to additional syngas, which additionalsyngas is recycled to a Fisher-Tropsh reactor used to produce saidliquid fuel.
 15. The method of claim 1, wherein said synthesis gas isintroduced into a combustor and heated to an inlet temperature of saidturbo-expander.
 16. A method for co-production of fuels, chemicals, andelectric power comprising the steps of: compressing a mixture comprisingan oxidant and CO₂ using a turbo-compressor, producing a compressedmixture; introducing said compressed mixture and a fuel into a partialoxidation reactor vessel, producing pressurized synthesis gas at a firstpressure; expanding said pressurized synthesis gas using aturbo-expander, simultaneously producing electric power and an expandedsynthesis gas at a second pressure less than said first pressure andgreater than one atmosphere; and converting said expanded synthesis gasto at least one of a product fuel and a product chemical.
 17. The methodof claim 16, wherein said oxidant is selected from the group consistingof air, oxygen-enriched air, oxygen, steam, and mixtures thereof. 18.The method of claim 16, wherein said first pressure is greater thanabout 60 psig.
 19. The method of claim 18, wherein said first pressureis greater than about 800 psig.
 20. The method of claim 16, wherein saidpressurized synthesis gas is introduced into a combustor prior toexpansion and heated to an inlet temperature of said turbo-expander.